1. Field of the Invention
The present invention relates to a platform for supporting at least one tranducer for reading from and/or recording on a data carrier. It is particularly applicable to magnetic reading and/or recording transducers for magnetic peripherals of data processing system, in particular disc memories.
2. Description of the Prior Art
The use of magnetic disc memories in data processing system is becoming increasingly popular because of their storage capacity and the relatively short time required to access data contained anywhere on the discs from the moment when the associated magnetic transducers receive an order from the processing system.
It is known that magnetic discs carry data on concentric, circular recording tracks whose radial width does not exceed a few hundredth of a millimeter and which generally cover the major proportion of both faces of the discs. The discs, which are parallelly and coaxially arranged, and usually of the same diameter, are driven in rotation at constant speed by an electric motor.
Current practice is generally to associate one or more transducers with each face of a disc, the number of transducers being very much smaller than the number of tracks. The tracks may be several hundred in number, but the number of transducers generally does not exceed approximately ten or twenty and is often equal to one. A plurality of transducers is used rather than only one when it is desired to achieve a substantial reduction in the time to access any item of data contained on on the face of a disc.
In a known manner, the transducers have electrical input and/or output wires connected to the electronic read and/or write circuits of the disc memory with which each transducer is associated. When the data recorded on face of a magnetic discs is to be read and the disc passes in front of the transducer or tranducers associated with this face, electrical signals are fed to the electronic read circuits via the electrical wires. Conversely, it will be clear that when the electronic circuits feed electrical signals to the transducer or transducers via the said electrical wires, data is recorded on the face of the associated magnetic disc.
In a known manner, transducers have a magnetic circuit provided with an air gap, around which a winding is arranged on to which is connected the electrical input and/or output wires. The air gap of each transducer is arranged opposite the face of the disc with which the transducer is associated, at a distance of a few microns, or even a few tenths of a micron so as to enable data to be read and/or recorded. It can therefore be said that the air gap constitutes the means by which the transducer picks up and/or records the data contained on the said face.
Current practice is for the transducers associated with one and the same face of a disc to be arranged on a single platform, which has to be moved radially with respect to the disc and above (adjacent) its face so that the transducers can access any item of data contained on the said face.
Such platforms are called "moving" platforms.
There are also known so-called "fixed" platforms which, during a read and/or write operation, remain at all times stationary above (adjacent) clearly defined tracks on the face of the associated disc. In such an arrangement, each transducer on a platform is associated with one and the same track.
Whether it is fixed or moving, the platform incorporating or supporting at least one transducer and associated with one face of a disc, is secured to a rigid removable arm. In a case of a moving platform, the arm is mounted on a carriage which moves externally of the discs, whereas in the case of a fixed platform the arm is mounted directly on the framework of the disc memory to which the disc belongs.
Regardless of the type platform, i.e. whether fixed or moving, the platform essentially comprises two parts, the "main body" and the suspension mechanism or means. The "main body" of the platform incorporates the transducers, and the suspension means has one end secured to the main body and its other end secured to the rigid removable arm.
Generally, but not exclusively, the main body of a platform is in the form a relatively shallow right-angled parallelepiped of which one major face, which for convenience is termed the "lower" face, contains the means or the transducers for picking up and/or recording data. The major face is for convenience termed the "upper" face and contains the ends of the electrical output and/or output wires of the transducer or transducers and the means which enable these wires to be connected to the electrical read and/or write circuits of the disc memory.
The terminology of "lower" and "upper face" will be continued throughout the specification; however, it should be noted that these terms merely signify a convenient positional relationship with respect to the platform and are not used in a limiting sense. Thus, they could equally as well be defined as first and second faces, minor and major faces or by any other suitable distinguishing description.
At the present time platforms known to those skilled in the art as "Winchester" platforms are becoming increasingly popular. Such platforms are manufactured by, inter alia, Applied Magnetic Corporation (A.M.C.) under the type numbers 3304 and 3306.
A platform of the "Winchester" type is produced as follows.
In the lower face of its main body are formed one or more gulleys, channels, grooves or the like whose depth is equal to or greater than 30 microns and may be as much as a few tenths of a millimeter. In the art these grooves are sometimes called bleed slots. This arrangement provides on the lower face a plurality of projecting portions which are called skids or longitudinal rails and which are, broadly speaking, ski-shaped.
In operation, as a result of the rotation of the discs, there is developed between the lower face of the main body of the platform and the face of the disc associated with the lower face of the platform, a cushion of compressed air which causes the platform to move away from the disc or rise, prevents the main body of the platform from touching the disc and thus damaging it. The platform is said to fly above the face of the disc with which it is associated.
The cushion of air exerts a pressure force on the surface of the skids or longitudinal rails in a direction generally normal thereto and directed from the lower face to the upper face of the main body of the platform. When in flight, dynamic equilibrium for the platform is achieved by setting up, in opposition to the force generated by the pressure of the cushion of air against the surface of the skids or longitudinal rails, a force termed a "loading force". The loading force is applied to the upper face of the main body and has a modulus equal to that of the pressure force.
It is known that the pressure force is proportional to the surface area of the skids or longitudinal rails. Thus, it can be appreciated that the presence of grooves or bleed slots in the lower face of the platform serves to reduce the surface area of the lower face adjacent a disc and enables the pressure force, and thus the loading force, to be reduced. The loading force is, for example, generated by a prestressed spring secured to the rigid arm carrying the platform. The force is relatively small and of the order of 10 to 20 grams. Because of this it is said that a platform of the "Winchester" type is a lightly loaded platform.
Generally, a platform of the "Winchester" type flies above the face of the disc with which it is associated in such a way that the surface of the skids or longitudinal rails is slightly inclined to the face of the disc. This allows the skids or longitudinal rails on the platform to produce sufficient lift to enable the platform to fly stably above the said face. The total surface area of the skids or longitudinal rails is defined as being the flight surface of the platform. The front of the flight surface is defined as being that part of the flight surface which is the first in time to have pass before it areas of the face of the disc with which it is associated. Similarly, the rear of the flight surface is defined as being that part of the flight surface which is the last in time to have these same areas of the face pass before it.
With magnetic transducers, the air gaps are arranged at the rear of the flight surface. The main body of the platform is inclined to the surface of the disc in such a way that the rear of the platform lies at a distance from the surface of the disc, termed the "altitude of flight". The distance is on the order of a few tenths of a micron and is less than the distance which separates the same face of the platform from the flight surface at the front. The distance of the front is of the order of a few microns.
Because of the fact that platforms of the "Winchester" type are lightly loaded, they enable low altitudes of flight to be achieved. Such platforms make is possible, by reason of their low altitude of flight, to obtain read signals from the transducer or transducers whose amplitude is relatively high, and allows reading of satisfactory reliability. Further, by reason of their light loading, the main body of the platform does not strike against the disc which action might damage both the disc and the main body of the platform.
However, the fact that platforms are lightly loaded does have certain disadvantages, the most important of which is that the flight of the platform above the face of the disc with which it is associated may be relatively unstable. To minimize instability, suspension of the platform should be arranged as close as possible to the face of the disc with which the platform is associated.
The current practice is for the device for suspending platforms of the "Winchester" type, which is generally formed by a resilient leaf spring, to be fixed at one end to the bottom of a groove formed in the upper face of the platform. The aim here is to maintain the point of attachment of the leaf spring close to the center of gravity of the main body of the platform with the object of improving the stability of the platform in flight. The other end of the resilient leaf spring is fixed to the rigid releasable arm which carries the platform. In this description, it will be assumed that the center of gravity of the main body of the platform virtually coincides with the center of gravity of the platform itself.
Thus, it can be appreciated that in a platform of the "Winchester" type, where the main body contains a groove formed in its upper face and a plurality of grooves or bleed slots formed in its lower face, these result in a reduction of rigidity or stiffness in the platform and an increase in its fragility. If the rigidity or stiffness of the main body of the platform is to remain adequate, it becomes necessary to restrict the depth of the groove containing the attachment for the leaf spring. Thus, there is a practical limit to the depth of this groove and it is not possible to move the suspension device sufficiently close to the center of gravity of the platform to avoid all problems connected with in-flight instability on the part of the platform.